The present invention relates generally to electrochemical cells, and especially relates to an electrochemical cell having an integral screen/frame assembly and/or a screen pack with a porous woven layer.
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells, including electrolysis cells having a hydrogen water feed. A proton exchange membrane electrolysis cell functions as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to FIG. 1, in a typical single anode feed water electrolysis cell 101, process water 102 is reacted at oxygen electrode (anode) 113 to form oxygen gas 104, electrons, and hydrogen ions (protons) 105. The reaction is created by the positive terminal of a power source 106 electrically connected to anode 113 and the negative terminal of a power source 106 electrically connected to hydrogen electrode (cathode) 107. The oxygen gas 104 and a portion of the process water 102xe2x80x2 exit cell 101, while protons 105 and water 102xe2x80x3 migrate across proton exchange membrane 108 to cathode 107 where hydrogen gas 109, is formed.
The typical electrochemical cell includes a number of individual cells arranged in a stack with fluid, typically water, forced through the cells at high pressures. The cells within the stack are sequentially arranged including a cathode electrode, a proton exchange membrane, and an anode electrode. The cathode/membrane/anode assemblies (hereinafter xe2x80x9cmembrane and electrode assemblyxe2x80x9d) are supported on either side by packs of screen or expanded metal which are in turn surrounded by cell frames and separator plates to form reaction chambers and to seal fluids therein. The screen packs establish flow fields within the reaction chambers to facilitate fluid movement and membrane hydration, and to provide both mechanical support for the membrane and a means of transporting electrons to and from the electrodes.
As stated above, the screen packs support the membrane assembly. The membrane is typically only about 0.002-0.012 inches in thickness, when hydrated, with the electrodes being thin structures (less than about 0.002 inches) of high surface area noble metals pressed or bonded to either side of the membrane and electrically connected to a power source. When properly supported, the membrane serves as a rugged barrier between the hydrogen and oxygen gases. The screen packs, positioned on both sides of the membrane against the electrodes, impart structural integrity to the membrane assembly.
Existing cell frames have a number of drawbacks and disadvantages. For example, current technology uses protector rings to bridge the gap between the cell frame and screen packs. The protector rings, typically positioned about the perimeter of the frame, prevent membrane extrusion and xe2x80x9cpinchingxe2x80x9d between the frame and the screen. Although these protector rings function well in operation, they render assembly of the cell very difficult, often breaking loose, resulting in misalignment and possible damage to the membrane. Specifically, because of their small cross-section, the protector rings tend to slide out of position and as a result often do not cover the gap between the frame and the screen which they are intended to bridge.
What is needed in the art is an improved screen assembly which provides structural integrity and simplified cell assembly while maintaining or improving the cell""s mass flow characteristics.
The present invention relates to an integral screen/frame assembly and to an electrochemical cell stack. The screen/frame integral assembly comprises: on: at least two screen layers, and a frame. Each of the screen layers has an interior portion with a porosity and has a periphery, a first screen layer having first openings having a size of up to about 0.077 inches (1.96 mm) or less b to about 0.033 inches (0.838 mm), wherein a thickness of said first screen layer is up to about 0.005 inches screen layer are interrelated such that the combination of said size and said thickness enable the passage of water and a gas through said first opening. The second screen layer has second openings, wherein the second screen layer is disposed parallel to and in contact with said first screen layer, wherein at least a portion of said second screen layer has a thickness grater than the thickness of the first screen layer. The frame comprises a frame material adapted to be extruded into the periphery of the screen while substantially maintaining the porosity of said interior portion, said frame having fluid conduits disposed therein, and wherein said frame material bonds said at least two screen layers together.
The electochemical cell stack comprises: an electrolyte membrane having a first gas side and a second gas side; a first gas electrode disposed on the first gas side of the membrane; a second gas electrode disposed on the second gas side of the membrane; and an integral screen/frame assembly disposed adjacent to and in intimate contact with the first gas side, comprising: one or more screen layers having an interior portion with a porosity and having a periphery; and a frame disposed about the periphery or the screen layer(s) while substantially maintaining the porosity of the interior portion, the frame having fluid conduits disposed therein.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.